Underwater devices are inter alia anchored bed stations or underwater vehicles such as AUVs (“Autonomous Underwater Vehicle”) and ROVs (“Remotely Operated Vehicle”). These are used in many areas of marine research and observation, for example for inspecting oil platforms, ships, pipelines, for support when laying deep sea cables, in the search for underwater mines, for measuring salt concentration and for exploration and mapping of the oceans.
An integrated energy store allows AUVs and bed stations to work completely autonomously, i.e. without a connection to a supply ship. Both systems are used at depths of a few meters to a few kilometers under the water surface.
In the case of AUVs, the average mission duration is a few hours to a few days. If surfacing is no longer possible during a mission, for example owing to a functional fault or an energy loss, a device is needed with which it is possible for the AUV to be salvaged from the sea bed or out of the ocean. One possibility consists in providing an apparatus with which the AUV can reach the water surface independently. This can be achieved for example by separating a weight from the AUV, as this increases the buoyancy of the vehicle.
A similar concept is used when salvaging bed stations. The typical structure of such an underwater bed station includes floats, measurement devices, an acoustic release and a weight. If the measurements to be carried out have finished, or if the energy store of the measurement device is depleted, a signal is sent from the measurement device or a supply ship to the release, which then cuts the connection to the anchor. The measurement device reaches the water surface owing to the buoyancy of the float situated on the top of the overall structure.
The separation of an emergency weights or a float can be realised in different ways. Documents U.S. Pat. No. 3,228,332, U.S. Pat. No. 3,463,113, U.S. Pat. No. 3,577,950, U.S. Pat. No. 3,848,226, U.S. Pat. No. 4,118,060, U.S. Pat. No. 4,136,415, U.S. Pat. No. 5,418,757 and U.S. Pat. No. 6,379,202 each describe mechanical release mechanisms, which are used for example to salvage bed stations. In these cases, the anchor to be decoupled is situated on a hook system, which is opened by an actuator.
In the cited patents, the hook is shaped in such a manner that the hook is unlocked after activation of the actuator and is then rotated or opened owing to a force acting on the hook, which results from the buoyancy force of the float. The release mechanism and the anchor must be in a defined position with respect to each other for this purpose. This is defined by the weight force of the load acting towards the earth's centre and the buoyancy force of the float acting in the opposite direction. In the case of a tilted position, the release mechanism can therefore fail.
In the known apparatuses, the arrangement of the hook system and actuator with respect to each other is considered disadvantageous, as the floats pulling on the bed station act constantly on the hook-opening mechanism with some of their buoyancy force. When the apparatus is activated, the actuator must therefore overcome different amounts of static and dynamic friction at the connection point between the hook and anchor depending on the size of the float and thus the buoyancy force it produces. With greater buoyancy forces, the opening mechanism therefore needs more energy.
U.S. Pat. No. 3,358,254 discloses a further mechanical release mechanism for underwater bed stations, with which the anchor can be decoupled by freeing a locking bolt. The locking bolt is held in its position by a rod element connected to an actuator until it is activated. If the actuator is activated, the rod element is moved out of its rest position and the locking bolt is freed. The fixing of the rod element in the rest position appears to be critical in this case. This is achieved by prestressing the rod element by means of a spring. This connection means that the release mechanism can be provided with only a limited buoyancy force. If the force of the float in the release direction exceeds that of the spring, there is a risk that the apparatus is opened. The actuator must in this case also be designed correspondingly, as it must overcome the spring force acting against it in order to open.
A further possibility for the design of a release mechanism is described in U.S. Pat. No. 3,609,825. In this case the load is held axially on a cylindrical apparatus by means of balls. This apparatus contains a movable piston, which forms a pressure chamber with the apparatus on one side and is exposed to the surrounding medium on the other side. With increasing depth, the pressure exerted on the piston by the water increases, as a result of which the piston is moved and the volume of the pressure chamber is reduced. If the piston reaches the position in which the balls which hold the load can slip into the openings present in the piston, the load is free is falls out of the apparatus. The problem in this case is the static and dynamic friction, which arises between the balls and the cylinder moving axially therebetween. The friction increases with greater weight forces and counteracts the axial movement of the cylinder. Finally, the release of the load is also connected to the attainment of a minimum depth. The load cannot be ejected at any desired depth. This restricts the functionality of an underwater vehicle considerably.
U.S. Pat. No. 3,749,933 discloses a release mechanism for bed stations, which decouples the bed station from the anchor by unlocking a closure. The closure is connected to an actuator by means of a mechanical transmission system, which reduces the force exerted on the closure by the floats as far as the actuator. The problem in this case is the high number of mechanical components of the transmission system used, which are exposed directly to seawater. There is a risk that the functioning of the mechanism is adversely affected by suspended particles and growth of vegetation. Furthermore, the force acting on the actuator, the magnitude of which depends on the coupled floats, could impair the actuator or result in inadvertent release.
U.S. Pat. No. 6,670,735 describes a release mechanism for an underwater vehicle. The emergency weight is ejected in that an actuator turns a spindle, as a result of which a bayonet closure is opened and the weight is freed. Mounted between the closure and the actuator is a series of gears, which reduce the rotation speed of the actuator. Furthermore, the weight to be ejected includes depressions with springs, which are intended to ensure the ejection of the weight. In this apparatus too there is a direct connection between the actual actuator and the load to be decoupled. The load thus exerts a force on the actuator and the gear stages at any time. If the release mechanism is activated, the actuator must overcome the static and dynamic friction arising owing to the load on all the components between the load and the actuator.
All the above-mentioned release mechanisms have a direct coupling between the actuator used in each case and the load used. The forces acting thereby on the whole release mechanism result in static and dynamic friction, which causes an increased energy requirement of the actuator. The energy requirement can also increase owing to suspended particles and organic growth which become stuck in the mechanisms. A further problem arising from the coupling consists in that the force which must be applied by the actuator depends directly on the load, as the actuator must counteract some of the force produced by the load in order to keep the apparatus closed. The load and the actuator must therefore be matched to each other, and only loads with little variation in weight or buoyancy can be used.
A release mechanism in which decoupling of the forces is effected is described in U.S. Pat. No. 3,063,395. In this case the actuator must apply a defined force. This is used for starting a mechanism which, once started, runs independently and results in ejection of the load. The load is completely decoupled from the actuator owing to a corresponding structure. The separation of the load is effected by the tightening of a Bowden cable, which frees a prestressed spring. The spring drives a hammer, which hits a cutter, which cuts the holding cable of the load. In this case the very complex structure and the high number of movable parts are viewed particularly critically. This results in a great susceptibility to faults, which is further increased by the likely accumulation of organic growth and suspended particles.
Further release apparatuses are known, with which a hook is used to hang up the load and on which hook torques act when it is in its closed position, in particular from the load itself, which requires the provision of additional securing mechanisms to prevent unintended release. Such release apparatuses are disclosed for example in U.S. Pat. No. 1,035,021, U.S. Pat. No. 4,136,415, U.S. Pat. No. 5,513,886 and FR 958,040.